The present invention relates to a powder feeding apparatus having an adjustable feed width and preferably a cross-feed auger. The cross-feed auger of the present invention maintains the powder feeding apparatus uniformly filled with a volume of powder to be dispensed for ultimately coating a continuous substrate or discrete articles. The adjustable feed width advantageously permits uniform coatings to be applied over different widths of substrates and discrete articles.
Electrostatic coating processes have been used to modify the surface characteristics of a substrate. In order to coat the substrate, a powder atomizer is combined with a feeder to deliver measured amounts of powder into an air stream. The air stream is directed to a coating apparatus, which electrically charges the powder particles so that they become attracted to the substrate. The powder is sometimes chemically highly reactive, and typically small in size. Strong electrostatic forces charge the powder particles and thereby cause them to be attached to the substrate. The substrate frequently is in continuous strip or web form, and advances continuously across or through the coating apparatus.
Electrostatic forces can be extremely strong on small particles, equaling perhaps 10 to 1000 times their weight. The electrode is often placed 4 to 6 inches away from the substrate to permit the vast majority of the generated powder dispersion to be diffused within that bound and thus beneficially influenced by the electrostatic effects. These include the electric field, ions created by the corona discharge energetically propelled by that field toward the strip, charge transfer by some of these ions colliding with the interspersed powder, and collision and momentum transfer between the energetic ions and the interspersed powder.
The powder dispensed from the powder feeder must be dispensed at uniform rates of flow; otherwise discontinuities or lack of uniformity may develop in the coating. The height of the powder within the powder feeder should be kept constant and level, in order to maintain a uniform head pressure at the feeder inlet. Should the substrate be disposed above the powder feeder inlet, then the substrate cannot be more widely spaced therefrom because the desired electrode stand off of 4 to 6 inches would not accommodate essentially all the powder flow between the first electrode and the substrate. Maintaining and controlling the volume of powder within the powder feeder has been difficult, because of the resulting limited height available between the substrate and the feeder.
In order to evenly distribute the powder onto the substrate, the powder should be evenly distributed across the powder feeder. The discharge rate is determined by the amount of powder that must be provided per unit time to coat the substrate at its transport velocity throughout its width to the desired thickness at a given deposition efficiency. Should the powder be non-uniformly distributed within the powder feeder, then the discharge rate from the powder feeder will not be uniform. Non-uniform powder discharge from the feeder will result in discontinuous or non-uniform coatings. Thus, there is a need in the art for an apparatus and method which functions to maintain a constant volume of powder throughout a powder feeder during operation of the electrostatic powder coater.
Previous attempts to solve the problem included shaking, blowing, levitating, and pushing the powder into the feeder. Shaking the powder along a transport path is disadvantageous, because an appropriate angle cannot be achieved in the limited space between the top of the feeder and the substrate for adequate feeding of the powder along tie range of discharge rates required to be attained and because of the strong tendency of shaking to agglomerate the powder. Blowing the powder into the powder feeder causes control over the amount of powder fed to the powder feeder to be lost, with the powder being non-uniformly distributed. Pushing the powder into the powder feeder may cause reactive powder to begin the onset of chemical changes, so that the powder will agglomerate or sinter prior to discharge and/or prior to application to the substrate. The use of a fluidization method to levitate powder in a slightly inclined trough through which the powder would flow laterally also has been attempted. This was not successful because the required inclination angle could not be obtained in the limited space between the feeder and the substrate and this method is unable to place the powder uniformly into the relatively wide brush feeder hopper across its width. Thus, there is a need in the art for an apparatus and method for maintaining a powder feeder uniformly filled, while minimizing the tendency of the powder to react.
This need has been addressed to some extent by an apparatus and method developed by Alexander et al., which was made the subject of a copending U.S. patent application Ser. No. 09/032,021 entitled Cross Feed Auger and Method filed on Feb. 27, 1998, now U.S. Pat. No. 5,996,855, the contents of which are incorporated herein by reference. Although the apparatus and method of Alexander et al. provide impressive results when the substrate has a width corresponding to the discharge width of the apparatus, the results are less than optimal when the substrate being coated has a significantly narrower width.
Narrower substrates centered in the apparatus, for example, leave substrate voids between the lateral end walls of the powder feeder and the substrate. Despite these voids, the powder feeder disclosed in the aforementioned copending patent application continues to disperse powder in the area of such voids. There is consequently a higher powder-to-substrate-surface ratio near the lateral edges of the substrate than there is near the middle of the substrate. This difference in powder-to-substrate-surface ratio tends to produce a non-uniform coating which is thicker at the edges of the substrate than it is near the middle. The non-uniform coating, however, is not the only disadvantage. The excess powder discharged into the lateral areas where it is not needed represents a waste of powder which increases material costs without any off-setting benefit.
There is consequently a need in the art for an apparatus and method capable of providing the advantages provided by the apparatus and method of Alexander et al., and also capable of providing those advantages regardless of whether a substrate significantly narrower than the width of the powder feeder is used. In this regard, there is a need in the art for a powder feeder with an adjustable discharge width.
Narrowing of the discharge width, however, without adjusting the width of the powder receptacle, will likely cause the powder at the ends of the powder receptacle to be fed at a rate which is different from that of the powder in the middle of the receptacle. Such non-uniform powder feeding would disadvantageously provide non-uniform coatings if the powder is used in a coating apparatus. There is consequently a need in the art for a powder feeder having an adjustable discharge width and also having a powder receptacle of adjustable width. Such an arrangement would permit adjustments of the receptacle width to be made in a corresponding manner when any adjustments of the discharge width are made.